Electroless gold plating methods have been of particular interest to the industry for many years and much has been written about the subject. It is also the subject for a number of patents in the United States as well as in foreign countries. A comprehensive review of the electroless gold deposition process has been published in the "Gold Bulletin" 1984, Volume 17, No. 4, pages 118-127. This paper is a literature study and a technical evaluation of published literature on this subject matter. The paper also contains a great many references including United States and foreign patents relating to electroless gold deposition. The paper points out that the prior art processes are not considered to be suitable for continuous production on a commercial scale, but some of the processes can be utilized in small scale applications with consistent success. Among the problems that limit the commercial success of electroless gold plating are:
1. Control of free cyanide, PA1 2. Methods of replenishing gold, PA1 3. Sensitivity to organic contamination, and PA1 4. Sensitivity to nickel contamination. PA1 1. Plating speeds of about 50 microinches per hour or better PA1 2. Stable solutions which will not spontaneously precipitate gold, even with extended use at high temperatures, PA1 3. Gold replenishments can be made with gold cyanide complexes, without fear of over stabilizing the bath due to excess free cyanide, PA1 4. Stability of the plating bath in the presence of nickel contamination, PA1 5. Stable solutions capable of at least three or more gold turnovers, PA1 6. The ability to plate 100 microinches or more, as required, and PA1 7. Relative simplicity of bath formulation, operation and control.
The prior art baths that are the most successful contain cyanide complexes of monovalent or trivalent gold, an alkali salt, some free cyanide, and a boron-containing reducing agent. Other additives are also recommended in some prior art baths, such as a buffering compound, a secondary complexing agent, or a chelating agent. It has been noted that one of the most serious contaminants that destabilizes the plating bath is nickel, which exists in the bath as a nickel-cyanide complex. The effect of nickel is to act as a catalyst to cause the spontaneous and rapid deterioration of the boron-containing reducing agent, with reduced plating rates as a result.
Free cyanide is difficult to control in these prior art baths. Cyanide can build up to high concentration levels which over-stabilizes the bath, thereby also reducing the electroplating rate. If gold is continuously replenished in the form of a cyanide complex, then free cyanide can build up rapidly due to the addition of cyanide contained in the gold cyanide complex. Attempts have been made to control this build-up by adding gold in the trivalent state, or in the form of alkali gold hydroxides, or alkali gold aurates, or as gold complexes other than cyanide. Gold added in these forms can control the cyanide build-up by absorbing the excess cyanide in controlled amounts. Although this solution provides one method for controlling free cyanide, other problems are presented, such as cost, availability, and solubility of the non-cyanide gold compound in the plating bath.